Retro-modulator phase conjugate mirrors (RM-PCM) have been proposed in the past as a means of extracting information from remote sensors. FIG. 1A discloses an ordinary mirror 106 and FIG. 1B discloses a phase conjugate mirror 114. In an ordinary mirror 106, an incident laser beam 104 from a source 102 is reflected at an angle when it strikes the ordinary mirror 106 to form a reflected beam 108. FIG. 1B discloses an incident laser beam 112 from a source 110 striking the phase conjugate mirror 114, the resulting reflected light 116 retraces the original path back to the source 110. Phase conjugate mirrors have the unique property that the light reflected back to the source must exactly retrace its path. Consequently light reflected from a phase conjugate mirror can remove deletrious wavefront aberrations such as those due to small scale atmospheric turbulence as well as provide an automatic pointing and tracking function.
In order to produce a remotely interrogated phase conjugate communication link, the following sequence of events may occur as shown in FIG. 2. First, the probe beacon 232 from a source 200 illuminates the general area of a sensor 202 having a RM-PCM 202a with a broad beam. The RM-PCM 202a is an optical, passive device. Second, the RM-PCM 202a generates a retroreflected beam 236 by self-pumped phase conjugation, establishing a communication link (comlink) between the source 200 and the sensor 202. Third, the data 240 to be transferred from the sensor 202 is encoded on the return beam 236 by modulating the phase conjugate reflectivity of the RM-PCM 202a. The wavefront of the incident beam 232 is reversed or phase organized to produce the retroreflected beam 236. Fourth, the retroreflected beam 236 propagates back to the source 200 substantially retracing its path, correcting wavefront distortions, and providing automatic pointing and tracking. The retroreflected beam 236 reaches the source 200 where a beam splitter 238 intercepts the retroreflected beam 236, the output of the beam splitter 238 is decoded it in a decoder 242 and the data 244 is retrieved.
Temporal encoding of the RM-PCM permits a high signal to noise communications link to be established. Most low power nonlinear optical phase conjugation systems proposed for communication links are based on photorefractive effects in crystals. These methods often require mutual coherence between the signal (probe) beam and the pump beams and generally employ self-pumped non-collinear degenerate four-wave mixing configurations.
The angular rate of tracking between a mobile beacon and a stationary interrogated sensor is roughly the ratio of the system angular resolution (ΔΘ=λ/d) to the response time of the nonlinear phase conjugate element. Although low power phase conjugation with self-pumped photorefractive crystals can be useful in many applications, it suffers from the major limitation that the power transmitted in the retroreflected beam will always be a very small fraction of the probe beam, a large amount of probe beam power will be needed to initiate the link, and since the response time of photorefractive systems are inversely proportional to the incident intensity, the link will be limited to extremely low data and tracking rates (sub-kiloHertz (kHz)). For configurations that phase conjugate the retroreflected beam at the probe transmitter, more moderate laser powers can be used, but multiple round trips between the probe beacon location and the sensor must take place to establish a solid link. While operating powers can be relatively low in this configuration, a higher power probe beam is generally required to initiate the link. In addition, the laser coherence length must be greater than or equal to the pathlength to the sensor, making stable narrow linewidth laser operation a requirement for long range operation. Alternate photorefractive geometries based on mutually pumped phase conjugation can mitigate coherence requirements but can be substantially more complex and still suffer from inherent photorefractive response time limitations.